In the production of fabrics utilizing ultrasonic energy, a wide variety of products can be produced. One particularly useful technique for the production of fabric ultrasonically is to form a non-woven fabric from a batt of primarily thermoplastic fibers. For the practical production of such webs it is necessary to form the fabric "full width," and to accomplish that it is necessary to provide a plurality of ultrasonic vibration sources disposed to act upon substantially the entire width of the web, the web width necessarily being greater than the width of a single vibration source under present technology. When forming the non-woven webs utilizing a plurality of ultrasonic vibration sources (particularly for a staggered configuration of vibration horns such as shown in U.S. Pat. No. 3,733,238 the disclosure of which is hereby incorporated by reference herein) there is a tendency for the non-woven fabric to be formed with a plurality of streaks extending in the longitudinal direction of the web. The streaks are clearly visible to the eye both before the fabric has been finished and subsequent to finishing, and render the fabric commercially unacceptable for many end uses.
The nonwoven fabric bonding process according to the present invention is distinct from the more conventional ultrasonic quilting process as describedin U.S. Pat. No. 3,733,238. The coverage of the "bond points" in a quilting process is substantially less than in the nonwoven fabric bonding according to the invention. The surface of the quilting material is a consolidated sheet rather than loose fibers that can be moved around. The high density of bond (or welding) points and the movable fibers contribute to a condition whereby "streaking" can easily occur, which streaking is desirably avoided according to the invention.
According to the present invention a method and apparatus are provided for producing a non-woven streakless web. According to the present invention the source of the streaking problem has been identified and apparatus and method techniques are provided to remedy the problem.
According to the present invention it has been determined that the streaks apparently are created due to a migration phenomena occurring as the fiberous batt passes beneath the vibration sources, particularly where first and second staggered rows of overlapped ultrasonic horns are provided and the web passes beneath the horns. When the web passes beneath the first row of horns and is bonded, the unbonded and relatively raised remaining strips between the horns of the first row experience some migration, flow, or movement of fibers therein to overlap slightly into the bonded strips along their adjacent edges. This overflow of fiber from the unbonded strips into the bonded strips results in a surplus of fiber along the edges between the bonded and unbonded areas so that when the web passes beneath the second row of horns and is bonded a relatively raised portion or "streak" is produced.
According to the present invention a method and apparatus are provided for producing a streakless bonded non-woven fabric by preventing significant migration of fibers in the fibrous batt just prior to bonding. This is accomplished by applying a compressive force to the fibrous batt web just prior to passage thereof into operative engagement with the vibration sources, as by providing a roller of hard material just before the vibration sources. If desired, the roller may be heated (e.g. to about 250.degree. F.) to more permanently deform and compress the fibers before they are led to the vibration sources. Migration can be further prevented by providing another compression roller upstream of the first roller, as between a vacuum screen and conveyor belt transporting the fibrous batt.
It is the primary object of the present invention to produce a substantially streakless bonded non-woven web from a batt of primarily ultrasonically fusible fibers. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, from the appended claims.